1. Field of the Invention
This invention relates to an implant for insertion between a person's tibia and talus to enable the ankle to function substantially normally and overcome or alleviate the effects of arthritis or avascular necrosis in the ankle joint.
2. Prior Art
The Buechel-Pappas Total Ankle Replacement System comprises:                a tibial component with an upwardly-projecting fixturing stem that extends centrally up into the patient's tibia in the marrow region, and a bottom plate located just below the tibia and presenting a flat bearing surface on the bottom;        a meniscus bearing component with a flat bearing surface at the top that slidably engages the tibial component plate from below, and a bottom bearing surface of compound curvature; and        a talar component with a top bearing surface of compound curvature substantially complementary to that of the bottom surface of the bearing component, and a pair of downwardly-projecting fixation fins on the bottom for reception in corresponding channels prepared in the top of the patient's talus by the surgeon.        
The Scandinavian Total Ankle Replacement (STAR) is broadly similar to the Buechel-Pappas system but differs from it in the compound curvature interface between the bottom of its bearing component and the top of its talar component, as well as in the mode of attaching its tibial component to the patient's tibia., which in the Scandinavian system is by means of attachment members of stepped cylindrical configuration on the top of the tibial component.
In the Buechel-Pappas and STAR implants the compound curvatures of the respective bearing and talar component interfaces tend to inhibit the natural movements of inversion and eversion in the ankle joint. In addition, some patients have experienced loosening of the tibial component of the implant.
The Agility™ Total Ankle system has its bearing component affixed to the tibial component and presenting a generally rectangular, round-cornered recess on the bottom which is substantially flat across most of its extent for snugly receiving a complementary talar component. Both the tibial component and the talar component have fixturing stems for implantation respectively in the patient's tibia and talus. This procedure requires resectioning a relatively large amount of bone, which may cause bone weakening and lead to bone fracture.